1. Field of the Invention
The present invention relates in general to a fuel reformer utilizing a metal thin film for use in a fuel cell driven mobile vehicle which makes use of, as a fuel, an alcohol such as methanol, ethanol, etc. More particularly, the invention relates to a miniature fuel reformer and a system thereof for use in a fuel cell driven mobile vehicle, which enables a reduction in size by adopting an integrated hydrogen generation/separation device in which the steam reforming reaction for the hydrogen generation, and the separation reaction for the hydrogen separation, are carried out simultaneously in the same reactor. The invention utilizes by the metal film property which selectively permeates only hydrogen of a reforming gas containing hydrogen, and by the principle by which the reaction rate increases beyond its equilibrium limit.
2. Description of the Prior Art
In general, a fuel cell system by which chemical energy converts directly to electric energy by a chemical reaction between a hydrogen gas and an electrolyte, is used where internal combustion engines are not substantially utilized, such as in the case of spaceship. Fuel cell systems are also used for supplying electric power to electric driven mobile vehicle. In particular, in recent years, the importance of the fuel cell system is increasingly highlighted due to the tightening of restrictions on air pollution, and the limitation of the development of electric vehicles using battery power.
Moreover, with increasing interest in energy efficiency and the environmental pollution problem, it is accepted that internal combustion engine driven vehicles can be virtually replaced with electric vehicles using fuel cells. However, there are various technical limiting factors that need still to be resolved, before these fuel cell driven vehicles can be widely commercially available. In particular, in a fuel cell driven vehicle that uses hydrogen gas as a fuel, all the problems associated with the construction of infrastructure for the storage, delivery, and supply of the hydrogen fuel act as limiting factors in the wide utilization of such vehicles.
Recent progress has been made in the development of a fuel reformer, wherein a liquid fuel such as methanol, ethanol, gasoline, and the like is reformed to generate hydrogen, which is then separated for utilization as a fuel. However, in order for such a fuel reformer to be widely utilized, developing a fuel reformer having smaller size, lighter weight, and excellent response to load is necessary. Particularly significant is that the fuel reformer is made smaller and lighter so that it can be mounted on the vehicle.
A Solid Polymer Electrolyte Fuel Cell (hereinafter, called “SPEFC”) has been used in recent fuel cell vehicles. The SPEFC, however, has a shortcoming in that it easily loses its activity by virtue of CO contained in hydrogen gas, that is, the fuel. For this reason, it is significant to lower the concentration of CO to a minimum.
Furthermore, to develop a fuel cell vehicle using a liquid fuel, thereby resolving the above described problems with respect to the use of hydrogen gas fuel, a process for preparation of hydrogen that utilizes the prior art steam reforming reaction has been developed worldwide. However, this technology has a problem in decreasing the total size of the fuel reformer system, because the steam reforming reaction and the hydrogen separation reaction are carried out in different reactors as shown in FIG. 1.
For example, in U.S. Pat. No. 4,613,436, an effort has been made to decrease the size of a hydrogen separation apparatus by arranging a plurality of vertically spaced circular hydrogen separation membranes while interposing airtight protrusions between adjacent membranes such that spaces defined between adjacent membranes by the airtight protrusions are isolated from one another. The reforming gas flow is passed through each of the hydrogen separation membranes to separate hydrogen, which is then captured. However, the structure of this hydrogen separation apparatus is very unsuitable for mass production, and also has difficulty in maintaining airtightness at its fixed portions. To resolve these problems, U.S. Pat. No. 5,536,405 proposes inserting gaskets between the hydrogen separation membranes during the arrangement of the membranes such that the resultant apparatus may be suitable for mass production, and such that airtightness may be easily maintained between the hydrogen separation membranes. A drawback of the apparatus in the latter patent, however, is its heavy weight due to the application of the gasket means.
Meanwhile, an attempt to improve the performance of the fuel reformer has been made. U.S. Pat. No. 5,458,857 attempted to carry out, in separate regions, hydrogen gas generation by the endothermic steam reforming of methane, and the exothermic transition reaction for transition of carbon monoxide and steam to carbon dioxide and hydrogen, so as to improve the efficiency of the fuel reformer, thereby realizing the miniaturization of the fuel reformer. However, the problems associated with this patent are that the necessary heat cannot be supplied by the heat from the transition reaction alone, and that the structure of the fuel reformer is complicated such that the efficiency is decreased. Furthermore, U.S. Pat. No. 5,741,474 attempted to simultaneously practice, in a certain space, the partial oxidation of methane, as an exothermic reaction, and the reforming reaction, as an endothermic reaction, thereby maximizing the heat transfer efficiency. However, there is a drawback in that both reactions cannot be efficiently controlled, thereby reducing the performance of the fuel reformer. Japanese Patent Application No. 93-147902 attempted to supply the necessary heat for the reaction by re-using, as a fuel, some of the hydrogen generated by the reaction of methanol with water, so as to improve the performance of the fuel reformer, and to miniaturize the fuel reformer as well. However, there is a problem in this design in that a separate device is necessary for initial heating, thereby limiting the miniaturization of the fuel reformer.
The above described inventions are characterized by limitations on performance improvement and fuel reformer miniaturization, because improvement in performance and miniaturization have been based on improvement of the fuel reformer structure, without resolving the prior art problem that the fuel reformer and the hydrogen separation apparatus must be separately present.
In an attempt to overcome such limitations, U.S. Pat. No. 5,888,273 discloses a hydrogen generation device in which a long tube-shaped membrane for separation of hydrogen and a steam reforming catalyst are positioned in the same space. Hydrogen gas is separated, and some of the reforming gases, containing hydrogen, carbon monoxide, carbon dioxide, steam, etc., are transported to a separate location, and then are burned, thereby supplying the necessary heat for the reforming. However, such a device is problematic in that initial ignition is difficult. Another problem is that the heat transfer efficiency of the device in supplying the necessary initial heat for the start of the reforming reaction, and heat for maintaining the necessary membrane temperature for the permeation of hydrogen, is poor. These problems are considered significant drawbacks against the fast initial starting period and dynamic response of the fuel cell driven vehicle. In addition, because of the complicated structure and the use of the long tube-shaped hydrogen separation membrane, the hydrogen generation device of U.S. Pat. No. 5,888,273 is unsuitable in mass production processes, such as in the manufacturing of vehicles.